Ruthenium-Catalyzed Synthesis of Cyclic and Linear Acetals by the Combined Utilization of CO2, H2, and Biomass Derived Diols

Article abstract of DOI:10.1002/chem.201901660

Herein a transition-metal catalyst system for the selective synthesis of cyclic and linear acetals from the combined utilization of carbon dioxide, molecular hydrogen, and biomass derived diols is presented. Detailed investigations on the substrate scope enabled the selectivity of the reaction to be largely guided and demonstrated the possibility of integrating a broad variety of substrate molecules. This approach allowed a change between the favored formation of cyclic acetals and linear acetals, originating from the bridging of two diols with a carbon-dioxide based methylene unit. This new synthesis option paves the way to novel fuels, solvents, or polymer building blocks, by the recently established “bio-hybrid” approach of integrating renewable energy, carbon dioxide, and biomass in a direct catalytic transformation.

Full text of DOI:10.1002/chem.201901660

A Journal of
Accepted Article
Title: Ruthenium-Catalyzed Synthesis of Cyclic and Linear Acetals via
the Combined Utilisation of CO2, H2 and Biomass Derived Diols
Authors: Jürgen Klankermayer and Kassem Beydoun
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To be cited as: Chem. Eur. J. 10.1002/chem.201901660
Link to VoR: http://dx.doi.org/10.1002/chem.201901660
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Ruthenium-Catalyzed Synthesis of Cyclic and Linear Acetals via
the Combined Utilisation of CO2, H2 and Biomass Derived Diols
Kassem Beydoun and Jürgen Klankermayer*[a]
Abstract: Herein a transition metal catalyst system for the selective great current interest are cyclic acetals (CA), such as 1,3-
synthesis of cyclic and linear acetals from the combined utilisation of dioxolane, 1,3-dioxane, 1,3-dioxepane. These chemicals
carbon dioxide, molecular hydrogen and biomass derived diols is represent interesting solvents for numerous applications, as well
presented. Detailed investigations on the substrate scope enabled to as important building units for homopolymerization or
largely guide the selectivity of the reaction and demonstrated the copolymerization, resulting in polyoxymethylenes as water-
possibility to integrate a broad variety of substrate molecules. This soluble polymers.[6] The general and industrially used
approach allowed to change between the favored formation of cyclic methodology for the preparation of CA is based on the
acetals and linear acetals, originating from the bridging of two diols condensation of formaldehyde with glycols in the presence of
with a carbon-dioxide based methylene unit. This new synthesis acidic resin catalysts, affording the corresponding CA bearing the
option paves the way to novel fuels, solvents or polymer building 1,3-dioxy moiety (Scheme 1, A).[7]
blocks, via the recently established “bio-hybrid” approach, by
integrating renewable energy, carbon dioxide and biomass in a direct
catalytic transformation.
The catalytic utilisation of carbon dioxide (CO2) as renewable
carbon source enables in principal the optimized synthesis of
existing chemical building blocks or the creation of new
molecular structures.[1] Furthermore, the increasing availability of
non-fossil energy technologies opens unique possibilities to tailor
within this concept the interface of energy and material value
chains. Conclusively, the incorporation of renewable energy in
processes with the combined utilisation of CO2 and bio-based
carbon feedstock offers, in-line with the principles of green
chemistry,[2] unprecedented synthetic pathways to carbon
reduced fuels, chemicals and solvents, lately introduced as “bio-
hybrid” approach.[3]
Scheme 1. “Bio-hybrid” approach towards the synthesis of cyclic acetals CA
and linear acetals LA from the combined utilisation of diols, CO2 and H2.
Herein, a novel approach for the synthesis of cyclic acetals CA
via the utilisation of CO2/H2 for the construction of CH2-unit in
Recently, a new catalytic reaction provided the first example for
such a methodology and demonstrated the selective conversion
combination with various diols is described (Scheme 1, B).
of CO2 and renewable hydrogen to the formaldehyde oxidation
Additionally, the molecular catalyst system in combination with
level.[4] Moreover, when combined with renewable alcohols, this
selected diols enabled the synthesis of linear acetals LA, yielding
approach opens a novel synthetic pathway to various dialkoxy
large diol structures, originating from the bridging of two diols
with a carbon-dioxide based methylene unit. These molecules
are highly interesting, as they could be utilized as novel
monomers for polymerization reactions (Scheme 1, B).
methane ethers.[4a] The key to this development was the access
to a molecular catalyst system with exceptional activity and
thermal stability in hydrogenation reactions.[5] In detail, this “bio-
hybrid” transformation could be realized with the molecular
ruthenium-Triphos catalyst [Ru(triphos)(tmm)] (Triphos=(1,1,1-
Furthermore, the synthesis is adaptable and versatile, as various
8]
bio-based diol structures could be utilized.[1,
Moreover, the
tri(diphenylphosphinomethyl) ethane, tmm
=
trimethylene
largely different boiling points of the CA and LA facilitate down-
stream processing and separation via distillation.
methane) and aluminium triflate (Al(OTf)3) as Lewis acidic co-
catalyst, via the formation of methyl formate (MF) and methoxy
methanol (MM) intermediates.[4a] Based on this approach the
access to a larger family of acetals could be envisaged, thus
allowing to substantiate the novel concept. The compounds of
In the first set of experiments, the [Ru(triphos)(tmm)] complex
was investigated in combination with Al(OTf)3 as acidic co-
catalyst and 1,3-propanediol as prototypical diol and reaction
medium. Generally, the reactions were carried out at
a
temperature of 80 °C for 18 h with a 1:3 ratio of the CO2/H2
gases at a total pressure of 80 bar (pressurized at room
temperature) and the TON of the desired product is calculated
according to the following equation (n = moles).[4a, 9]
[a]
Dr. K. Beydoun, Prof. Dr. J. Klankermayer
Institut für Technische und Makromolekulare Chemie
RWTH Aachen University
Worringerweg 2, 52074 Aachen (Germany)
E-mail: jklankermayer@itmc.rwth-aachen.de
푛(푎푐푒푡푎푙)
푇푂푁 =
Supporting information for this article is given via a link at the end of
the document.
푛(푐푎푡)
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Using the [Ru(triphos)(tmm)] catalyst in 6 µmol and the Lewis dioxane as solvent.[11] Running the reaction for a longer time (40
acid in 25 µmol resulted in the formation of the envisaged cyclic h) resulted in increased formation of CA-1 (TON = 239) and the
acetal product 1,3-dioxane CA-1 with a turnover number (TON) minor production of LA-1 (TON = 100), hinting towards a
of 32 (Table 1, entry 1). In addition, the unexpected formation of subsequent intramolecular trans-etherification of LA-1 to CA-1
the linear acetal LA-1 could be identified (TON = 55). This (Table 1, entry 8). More detailed information on the optimization
product formally results from the multi-component combination of of reaction conditions and parameters can be found in the
CO2, H2, and two 1,3-propanediol units. Decreasing the catalyst supporting information). Notably, the use formic acid as the C1
loading to 3 µmol significantly increases the total TON (322) of source for the construction of the acetal unit in CA-1 and LA-1
the acetal products, slightly favoring the LA-1 product over 1,3- was investigated and both products were formed in a total TON
dioxane (Table 1, entry 2). These results clearly indicate that the of 424, slightly favoring the formation of CA-1 over LA-1 (TON =
catalytic system can be optimized by tailoring of the additive to 245 and 179, Table1, entry 9).
catalyst ratio and catalyst loading.[5a, 10] Moreover, in absence of
Al(OTf)3 co-catalyst no acetal products could be detected.
In the subsequent investigations the influence of diol
modifications on activity and selectivity should be evaluated.
Consequently, ethylene glycol was used in the standard
transformation, resulting in the acetals 1,3-dioxolane CA-2 and
LA-2 (Scheme 2). CA-2 is of particular industrial interest, as it
represents an important component of industrial polymers and
can be used as solvent for polar polymers, leading to
applications as cleaner for epoxy and urethane products, where
its low boiling point helps achieve high throughput or fast
drying.[6] Using the previously optimized reaction conditions, CA-
2 was obtained with a TON of 147 in combination with the major
product LA-2 (TON = 475, Scheme 2). In this transformation the
effect of solvent was very pronounced and in a neat reaction
mixture increased selectivity towards LA-2 could be observed
(TON = 64 for CA-2 and TON = 514 for LA-2).
Table 1. Ruthenium-catalyzed synthesis of cyclic acetal CA-1 (1,3-dioxane)
and linear acetal LA-1 using CO2 and molecular hydrogen.[a]
CA-1
(TONb)
LA-1
(TONb)
Entry
Solvent/1,3-propandiol
t [h]
1c
2
(--/2)
18
18
18
18
18
18
18
40
18
32
55
(--/2)
137
13
185
42
3
DMSO (1/2)
4
2-MeTHF (1/2)
THF (1/2)
71
78
5
156
121
275
239
245
125
145
221
100
179
6
DEE (1/2)
7
1,4-Dioxane (1/2)
1,4-Dioxane (1/2)
1,4-Dioxane-HCOOH (1-0.2/2)
8
9d
Scheme 2. Ruthenium-catalyzed synthesis of 1,3-dioxolane CA-2 (as a solvent,
reagent, or monomer for different polymerization reactions) using ethylene
glycol with CO2 and molecular hydrogen
[a] Reaction conditions: Catalyst = [Ru(triphos)(tmm)] (3 µmol), Acid (25 µmol),
1,3-propandiol (2 mL), CO2/H2 (20/60 bar), 18h, 80 oC; [b] Turnover number
(TON) was determined by NMR-sepctroscopy using mesitylene as internal
standard; [c] [Ru(triphos)(tmm)] (6 µmol); [d] only H2 (80 bar).
This experimental observation and the comparable results
obtained when using HCOOH, lead to the proposal of a
sequential pathway for CA-1 and LA-1 formation starting from
1,3-propanediol, CO2 and H2 (Scheme 3). Thus, HCOOH will be
formed via hydrogenation of CO2, followed by the esterification
Subsequently, the effect of co-solvents was investigated, as this
modification enhances the dissolution of the reactive gases and
facilitates reaction control. The use of DMSO resulted in
diminished formation of CA-1 and LA-1 with minor TONs of 13
and 42 (Table 1, entry 3). Comparable results were obtained with
bio-based 2-methyltetrahydrofuran (2-MeTHF) and only TONs of
71 and 78 could be measured (Table 1, entry 4). Using
tetrahydrofuran (THF) and diethyl ether (DEE) resulted in the
improved formation of CA-1 with TONs of 156 and 121 and LA-1
with TONs of 125 and 145 (Table 1, entries 5 and 6).
Interestingly, when using 1,4-dioxane as co-solvent, the reaction
resulted in the formation of CA-1 and LA-1 with an increased
TON of 275 and 221 (Table 1, entry 7). This improved reactivity
of the catalytic system can be largely attributed to the higher CO2
solvation capacity of 1,4-dioxane and paves the way to a self-
breeding system with the application of the reaction product 1,3-
with 1,3-propanediol, yielding 3-hydroxypropylformate.
A
subsequent reduction results in the formation of the respective
alkoxymethanol intermediate, which is in an intramolecular
acetalization reaction forming CA-1. In a second pathway, the
alkoxymethanol intermediate can be condensed with another
1,3-prapanediol molecule, forming the linear acetal diol product
LA-1. Importantly, LA-1 can also serve as an intermediate
towards CA-1 formation via an intramolecular trans-acetalization
step, confirmed by the results with increased selectivity at longer
reaction times. Thus, a clear difference to the established
reaction with formaldehyde is revealed. With respect to the
catalysts system, the reduction steps are facilitated by the
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ruthenium complex, whereas the esterification and acetalization
steps are catalyzed by the Lewis-acidity of the multi-functional
catalytic system.[12]
Entry
1
Diol
Acetal (TON)b
2
Scheme 3. Reaction pathway for the [Ru(triphos)(tmm)]/Al(OTf)3-catalyzed
synthesis of CA-1 and LA-1 with 1,3-propanediol.
3
4
To evaluate the versatility of this approach for the synthesis of
advanced linear and cyclic acetals, the reactivity and chemo-
selectivity of reactions with structurally different bio-derived diols
was targeted and the decisive experiments are summarized in
Table 2. When 1,2-propanediol was used, the transformation
afforded 4-methyl-1,3-dioxolane CA-3 with a TON of 199 as the
major product and the linear acetal LA-3 with a TON of 68,
resembling a chemo-selectivity of 74 % (Table 2, entry 1). The
use of the sterically more hindered 1,2-hexanediol resulted in a
lower overall reactivity towards the acetal products, with 4-butyl-
1,3-dioxolane CA-4 obtained with a low TON of only 38 (Table 2,
entry 2). Interestingly, applying 1,3-butanediol, the acetal
products were obtained with a total TON of 330 and an
astonishing selectivity of 92% towards the 4-methyl-1,3-dioxane
CA-5 in correlation to linear acetal LA-5 (Table 2, entry 3).
Additionally, the use of the sterically more hindered 2,3-
pentanediol resulted in the formation of 4,6-dimethyl-1,3-dioxane
CA-6 in a TON of 115 and selectivity of 85% (Table 2, entry 4).
Remarkably, the selectivity of the acetal products was
significantly reversed when a diol with longer chain was used as
substrate. This becomes clearly evident with 1,4-butanediol,
affording the linear acetal product LA-7 as the preferred product
with a TON of 252 (86% selectivity, Table 2, entry 5).
Furthermore, the use of 1,5-pentandiol and 3-methyl-1,5-
pentandiol resulted in a selective formation of the corresponding
linear acetal products LA-8 and LA-9 with TONs of 151 and 342
respectively (99% selectivity, Table 2, entry 6 and 7) and only
trace amounts of the corresponding cyclic acetals were
observed. The clearly shifted chemo-selectivity in correlation to
the chain length of the applied diols (C2, C3 versus C4, C5)
enables to guide the formation of linear and cyclic acetals.
5
6
7
[a] Reaction conditions: [Ru(triphos)(tmm)] (3 µmol), Al(OTf)3 (25 µmol), diol (2
mL), 1,4-dioxane (1mL), CO2/H2 (20/60 bar), 18h, 80 oC; [b] Turnover number
(TON) was determined by NMR-spectroscopy using mesitylene as internal
standard.
Conclusion
Whereas current approaches largely focus on the combined
usage of CO2 and molecular hydrogen towards C1-synthons and
e-fuels, the present methodology enables to synthesize complex
molecular structures with the additional direct integration of bio-
based platform molecules (Figure 1).[13] Thus, the concept of
“bio-hybrid” transformations was introduced, envisioning the
integration of biomass and CO2 utilization. In the present work a
feasibility demonstration was anticipated and in addition to the
existing bio-hybrid synthesis of dialkoxy methane ethers,[4a] the
catalytic production of versatile linear and cyclic acetals via novel
synthetic pathways could be established with TONs up to 622.
Moreover, a detailed investigation on the substrate scope
enabled to largely guide the selectivity of the reaction and in
representative cases a chemo-selectivity >99 % could be
achieved. Thus, in these catalytic reactions the versatile catalytic
utilization of molecular hydrogen from renewable resources may
pave the way to novel processes with largely reduced carbon
footprint. Furthermore, the flexibility of the “bio-hybrid”
transformations also allows to utilize diols from polymeric waste
Table 2. Ruthenium-catalyzed selective synthesis of cyclic acetals CA and
linear acetals LA using variable diols with CO2 and molecular hydrogen.[a]
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and research on this topic is currently ongoing in our
laboratories.[5c]
exploration, validation and implementation of ‘Power-to-X’
concepts. We thank Khouloud Beydoun for preparing the cover
artwork and graphics. We also thank Umicore AG for a generous
gift of ruthenium precursor.
Keywords: molecular catalysis · carbon dioxide utilisation ·
catalytic hydrogenation · sustainable chemistry · green chemistry
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Figure 1. Bio-hybrid approach for incorporation of renewable energy in
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Experimental Section
General procedure for the synthesis of cyclic acetals (CA) and linear
[7]
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acetals (LA) using diols, CO2 and H2:
A 2.0 mL solution of
[Ru(triphos)(tmm)] (0.002 g, 3 µmol) and Al(OTf)3 (0.012 g, 25 µmol) in
the selected diol was prepared under argon atmosphere in a Schlenk tube
containing a stirring bar. 1 mL of 1,4-dioxane is added to the solution.
After stirring for 5 minutes, the solution was transferred to a carefully
degassed and dried 20 mL stainless-steel autoclave. The autoclave was
pressurized at room temperature with 20 bar CO2 and then H2 was added
up to a total pressure of 80 bar. The reaction mixture was stirred with a
magnetic stir bar and heated to 80 °C using a preheated aluminum cone.
After 18 h the autoclave was cooled in an ice bath and then carefully
vented. Turnover numbers (TONs) of desired product in solution were
analyzed by 1H-NMR spectroscopy using mesitylene as internal standard
with an estimated error of 6%.
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This work was supported in part by the Deutsche
Forschungsgemeinschaft (DFG, German Research Foundation)
under Germany´s Excellence Strategy – Exzellenzcluster 2186
„The Fuel Science Center“ ID: 390919832, and by the German
Federal Ministry of Education and Research (BMBF) within the
Kopernikus Project P2X: Flexible use of renewable resources –
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Pischinger, A. Bardow, Energy Environ. Sci. 2018, 11, 331-43.
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COMMUNICATION
Layout 1:
COMMUNICATION
Bio-hybrid Products: A transition
metal catalyst system for the
selective synthesis of cyclic and
linear acetals from the combined
utilisation of carbon dioxide,
molecular hydrogen and biomass
derived diols is presented. This
new synthesis option paves the
way to novel fuels, solvents or
polymer building blocks, via the
recently established “bio-hybrid”
Kassem Beydoun and
Jürgen Klankermayer*
Page No. – Page No.
Ruthenium-Catalyzed
Synthesis of Cyclic
and Linear Acetals via
the Combined
Utilisation of CO2, H2
and Biomass Derived
Diols
approach,
by
integrating
renewable energy, carbon dioxide
and biomass in a direct catalytic
transformation.
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